There is no official theory, but here's what I think: Rock was the most popular type of music in the world and everybody was listening to it. So the big US oil companies and the Western governments, which were not being very ethical during the booming the post-war years, thought that Rock would be a great way to send subliminal messages to the world in order to support them in creating the mass-consumption society. Therefore they financed these bands, or maybe just their producers, made sure the songs had some implicit messages and started brainwashing the listeners.
However, when the oil extraction and production declined in the US, there was less money to finance these bands, so the artists became more and more independent producing crap music without an actual purpose.
Yes, I love conspiracy theories.
We have to give our music a purpose so that it can brainwash us again with great tunes. We need big lobbies to get into the music business, and the cool subject of the moment is Climate Change. So we could have songs from the climate change lobbyists (You're heart is green) versus songs from the anti-climate change lobbyists (Let me pump all that blood out of your heart).
That's good stuff!